Vacuum tube



my-1m, i928. 1,677,850

E. Y. ROBINSON VACUUM TUBE Filed Nov. 27, 1922 2 Sheets-Sheet l E. Y.ROBINSN VACUUM TUBE Filed Nov, 27, 1922 2' Sheds-Sheet v y ra/25W:d/kyra@ Patented July 17, 1928.

UNITED STATES PATENT OFFICE.

ERNEST YEOMAN ROBINSON, OF MANCHESTER, ENGLAND, ASBIGNOB T0 IETBOPOLI-TAN-VICKERS ELECTRICAL COMPANY LIMITED, 0F LONDON, ENGLAND, A BRITISHJOINT-STOCK COMPANY.v

vacuum TUBE.

Application filed November 27, 1922, Serial No. 603,715, and in GreatBritain December 6, 1981.

This invention relates to vacuum electric discharge tubes havingelectron emitting cathodes therein such, or example, as the so-calledthermionic reotifiers and valves used in wireless telegraphy andtelephony and has for its object to provide an 1mproved construction andarrangement .of vacuum tube the envelope of which contains an ionizablerareiied atmosphere but 1n which ionization is eliminated or reduced toa minimum in a simple and eifective manner.

My invention comprises the combination with a cathode providing a,concentrated source of electrons and an anode in close proximitythereto so that the inter-electrode space is small, of means forconfining the space current to the said inter-electrode space.

My invention further comprises the provision of screens orshields forconfining the space current to the said inter-electrode space.

My invention further comprises the coniining of the space current to theinter-electrode space by reducing the activity for the emission ofelectrons of those portions of the cathode from which electron currentswould normally tend to pass outside the inter-electrode space.

My invention further comprises the 1mproved details of construction andarrangement hereinafter described and claimed.

The cathodes of valves constructed according to this invention are suchthat the electron source -is concentrated. Preferably, the activesurface of the cathode is continuous. It may consist of a sheet ofmetal, such as tungsten, which may be :Hat or curved to any desiredshape. It may, for example, be formed into a. cylindrical tube and beheated by a thermionic discharge from an auxiliary cathode disposedwithin the tube: or it may be heated by radiation from a heated filamentsimilarly disposed. lf the cathode be a flat sheet, then two such sheetsmay be employed with the auxiliary cathode or heating iilament betweenthem.

A cathode. the electron source of which is concentrated and whichapproximates in action to a cathode whose surface is continuous, mav beconstructed from a coiled lilament Whose turns are closely spaced.` Thecross section of the filament may preferably be rectangular, whichsection permits of The anode is preferably of a similar shape to thecathode and is so mounted that there is a. small gap which is uniformover the active face of the cathode, between the anode and cathode. Invalves constructed according to present practice, the amount ofionization is mainly determined by the electrons which travel b the.fringing paths because, firstly, the ionization is proportional to thelength of the path and, secondly, the resence of gas (and hence ofionization) re uces the impedance of the fringing paths to the spacecurrent more than it reduces the impedance of the direct paths so that alarger yproportionof the space current iiows by the fringing paths whengas is present. The effect is cumulative and hence in a soft valve themain portion of the current is carried alon the fringing paths, whichare paths o longest possible length and so the degree of ionization ismainly determined by the maximum length of path which is possible insuch a valve. Now in accordance with my invention, the space throughwhich the space current flows is reduced to a minimum by being limited(as `hereinafter more fully described) to the inter-electrode space inwhich the number of gas molecules available for ionization is limited.The electrons are highly concentrated in thisspace and so molecules ofgas may usually experience more than one collision before they reach thethe cathode and give up the charge they ac-y quired at the firstcollision. At the second collision, the charge on these ions may beneutralized so that an increase of space current will not result in anincrease of ionization for although more molecules are ionized when thespace current increases, more ions are neutralized before they reach thecathode. I have found that with a vacuum tube constructed in accor-dancewith this invention, the pressure of the residual gas in the tube duringoperation may be many times that permissible in valves constructed inaccordance with present practice before ionization becomes appreciable.

The space current may be confined to the inter-electrode space bydifferent means such as by suitably placed screens or shields which lostrode space. Or the edges and other portionsv prevent by actualmechanical screenin or by electrostatic screening, or by both mec anicaland electro-static screening, the flow oit space current outside thelimits of the inter-elecof the cathode from which electrons wouldnormall pass outside the inter-electrode space may be ept at asufficiently low temperature that they do not emit an a preciable amountof electrons. This may e effected by arranging that either less heatingcurrent is supplied to the said port-ions of the cathode or the said)ortions have a large and eiicient heat ra iating surface. Or acombination of these means may be employed. A further means of confiningthe space current to the inter-electrode space consists in making theportion or portions of the active surface `of the cathode from which theelectrons 'are emitted which tend to How by the fringing paths to theanode, of a material which has a llow electron emission co-eiiicient.Thus, with fa cylindrical tubular cathode having its active surface onthe outside, the tube may be made of tungsten and the outside surfaoecoated with thorium, the tube being operated at a temperature at whichthe electron emission from tungsten is negli- `ble. When the spacecurrent is thus coned, the inactive portions of the cathode lact aselectrostatic screens confining the space current to the inter-electrodespace. Those specific embodiments wherein the boundaries of the cathodehave relatively reduced activity for electron emission form thesubjectmatter of a continuing application Serial No.

96,050, filed March 19, 1926.

The accompanying explanatory diagrams illustrate the application of myinvention.

Figures 1 and 2 illustrate the eiect of decreasing the gap in anordinary diode.

Figures 3 to 9 illust-rate several applications of my invention.

Referring in the rstplace to Figure 1, in which ais a filament cat odeand b a tubular anode, it will beseen that electrons will travel alongdirect paths as c and indirect or fringing paths as d and e. The lengthofthe path e will be limited by the valve envelope (not shown). Theratio of the space current which Hows along paths such as c to thatwhich iiows along fringing paths such as d and e is proportional to theratio of the direct electrostatic ux to the fringing flux. It will thusdepend on the gap between the electrodes and the area of the electrodes.If the gap between the electrodes be reduced, the ratio of the directflux to the 4fringing iiux will be increased, but the ringing lux willstill remain. As previously stated, if ionization be present, the spacecurrent tends to flow more and more by the fringing paths. The amount ofionization in a valve is determined mainly by the length of the longestvpath.

navigano Figure 2 shows a valve tube in which the anode consists of twoparallel plates with a filament cathode between them. Considering asingle filament, the direct and shortest path for electrons is at fwhilst fringing currents will flow at g and L. The latter path isrelatively long and when the gapbetween the electrodes is of the ordersay of 1 cm. it will not be appreciably reduced by shortening the gap. Areduction in gap between the electrodes will, when the gap is small,have less eilect in reducing ionization with this arrangement ofelectrodes than with the arrangement shown in Figure 1.

Figures 3 and 3 illustrate the a plication of the present invention to arecti er having a cylindrical cathode. The latter has the referenceletter z' and constitutes a concentrated source of electrons which is anessential feature of my invention. It is heated by a discharge from anauxiliary filament cathode y'. z' is placed inside the cylindrical anodela which is mounted coaxially with and at a very small distance Afromthe cathode. At the opposite open ends of the cathode j, I provideshielding screens lmJ and 'n which may be metal discs of as large adiameter as possible (without touching the anode) in order that they mayexert amaximum screening eii'ect. The screens m and n are, in theexample illustrated, insulated from the main cathode z' and electricallyconnected to the auxiliary cathode g so that they are at negativepotential with respect to the cathode. If the gap between the discs orshields and anode is small, the discs may be made thin, their thicknessbeing increased with increase of the said gap. The cathode is supportedby three stems o which pass through the shield m. The auxiliary cathodesupport p passes through the shield fn.

The shields 'm and n prevent electrons from the edgesand inside of thetube z' from flowing to the anode 7c and conne the electrons flowingfrom the active surface of the cathode z' (in this case the outer'surface of the tube), to the inter-electrode space. The cathode isheated by a thermionic discharge from the auxiliary cathode g, in thiscase a closely coiled wire helix (which may preferably consist of thesame material as the cathode) in order to provide a device having aconcentrated source of electrons and a small inter-electrode spacebetween the cathode and the auxiliary cathode.l '.The auxiliary cathodevg is heated by an electric current, and the main cathode is madepositive to the 'auxiliary cathode so that a space current flows fromthe latter to the main cathode, which space current heats 'the cathodein a manner itself well known. The heating space current is confined tothe inter-electrode space between the auxiliary cathode and the cathode,and fringand n which thus'serve a dual purpose. In the example shown,the auxiliary cathode is supported by the stems p and r. yThe shieldsare attached to the stem p which provides a convenient method ofcharging them negative to the cathode. Alternatively, the shields m-andn ma be insulated. They may still be supporte by the stem p but may beaffixed thereto by means of insulating beads, constructed of a materialsuch as, for example, quartz, in a similar manner to that by which thegrid is supported as hereinafter described with reference to Fig. 7. Inthis case, they must be attached to an extension of the ,stem p, whichextension is so designed that at the point of attachment of theinsulating beads, it is relativelycool.

If desired, the cathode may be heated entirely by heat radiated from afilament g, the shields being so designed4 or arranged that they do notbecome heated to a temperature at which they emit electrons appreciably,this result being obtained if the surface area of the shields issufliciently large as can be determined by experiment.

Where tubular electrodes are employed the shield may consist of a tubewhose length is preferably at least twice its diameter, and whosediameter is as large as possible without the shield touching the anode.

Figure 4 shows an example of the application of this invention to arectifier having plane electrodes. For clearness, the anode is omittedin the drawing. The cathode consists of two plates of suitable materialsuch, for instance, as tungsten, mounted parallel with and facing eachother. They are heated by a discharge from the auxiliary cathode formedby the heated filament t. This filament is held taut by the springsupports u, and the space betweenY the adjacent layers of this filamentis reduced to as small a Value as possible, in order to provide aconcentrated source of electrons, so that the heating space current mayflow without producing appreciable ionization. The space current fromthe cathode to the anode is confined to the inter-electrode space by theshield o which is placed round the edges ot the cathode. It is separatedby a small gap from the cathode, in order to insulate it; it ispreferably joined electrically to the negative end of the auxiliarycathode. The shield contains slits such as 'w which permit the filamentsupports to pass through. This shield also acts as a shield to theheating space current and confines it to the interelectrode spacebetween the auxiliary cathode andthe cathode., The anode consists of twometal sheets (not shown) mounted parallel to and facing the plates 'wand y and is separated from them by a small gap. The cathode plates maybe heated by radiation from the filament in lieu of a discharge from theauxiliary cathode in a similar manner to the case when cylindricalelectrodes are employed.

Figure 5 shows an application of the invention using a filament cathode.The anode is shown at k; the cathode z' consists of a closely spacedhelix of tungsten wire supported by the leading wire 1', the other endbeing attached to the shieldin screen m which may preferably consist o atungsten or molybdenum disc. This screen is fastened to the rod p.' VTheshielding screens aa shown at m and n, the screen n being attached tothe supporting rod p and pierced y to allow the entr of the tungstenwire attached to the lea ing wire r. The cross section of the filamentmay be circular or rectangular.

Figure 6 shows an application of the invention to a valve havingcylindrical electrodes which is more able to stand the high potentials.In Figure 6, the anode is shown at 7c. The filament z' is wound in azig-zag path, each turn lying approximately on the surface of animaginary cylinder and substantially parallel Vwith the axis of thelatter and the turns are supported at each end by spring spiders 2, eachof which is constructed of wire fused into an insulating bead 3 such as,for instance, quartz. In the figure the filaments and the supportingspiders at one side of the plane of the section are shown. The quartzbeads 3 are supported by the metal rod p. The adjacent layers of thefilament are arranged as close as practicable to each other. The screensare, shown at m and n. They are supported by and connected to the rod p.v

Figures 7 and 7'r1 show an example drawn to an enlarged scale ot' theapplication of this'invention to a three electrode tube havingcylindrical electrodes where the cathode is heated by a thermionicdischarge from a form of discs m and n attached to the stem p. The discn is pierced to allow the entry of the lead' in wire r, and the cathodesupporting stems o. The grid is composed of substantially parallel wires5 supported by the two rings 6 and 7. In the case shown, the grid wiresyare held taut by the springs 8. The annular ring 6 is slotted to permitthe grid wires to pass through to the supporting springs. The rings 6and 7 are supported by the stems such as 9, fused into the insulatingbeads l0.' The annular rings 6 and' 7 serve as auxiliary screens toconfine the space current to the inter-electrode space, and they arepreferably mounted so that the gap be,- tween them and the anode is assmall as possible. Fig. 8 shows an application of the irivention to avalve employing a c'athode'having its outside surface coated withthorium and vwherein the main space current is confined to theinter-electrode space by making the boundaries of the ycathode of amaterial which has a low electron-emission coeiiicient. In theconstruction shown in this figure, z" represents the main cathode whichcomprises a cylindrical tube composed of tungsten the outside srfac ofwhich is provided with a coating 2 of thorium which coating howeverterminates short of the ends of the tube, as shown, so that the tungstencomposing the tube will be exposed at these boundaries of the cathode.The auxiliary cathode j', and cylindrical anode 7c correspond with theslmilarly designated parts shown in Fig. 3, the shields m and n preventelectrons from the inside of the main cathode 'i' from flowing to theanode 7c', and the thoriated outside surface of the main cathode i isopposite to and arranged within the anode which is spaced from theactive surface of the main cathode substantially as described .inconnection with Fig. 3. In using a valve made in accordance with thisembodiment of the invention, it is operated at a temperature at whichthe electron-emission from the uncoated tungsten ends or boundaries ofthe main cathode is negligible. These inactive boundaries of the cathodethus act as electrostatic screens which confine the space currentbetween the cathode and anode to the inter-electrode space. l

Fig. 9 is a diagrammatic view showing an application of the invention toa valve ode is attached to two co-axial heat radiating lead-in wires 1"and it is only so much longer than the anode k2 that the ends of thecathode are cooled by the lead-in wires r to such an extent that theelectron-emission from said end portions of the cathode is small or evennegligible compared with that from the main or central portion of thecathode. The temperature distribution along the cathode is typicallyindicated by the curve X, and the corresponding electron-emission lisindicated by the curve Y. The portions of the cathode from whichelectrons would normally pass outside the inter-electrode space are thuskept at a sufficiently low temperature that they do `not emit anappreciable amount of electrons.

In the construction of vacuum tubes according to this invention, thebest results with a given type of cathode and shields and a constant gaspressure, are obtained when the inter-electrode space is smallest.

For a ltube to have hard characteristics the gap must be reduced as thepressure of the I residual gas is increased; it must also be reduced asthe molecular weight of the residual gas is increased. The gap necessaryto give a tube of hard characteristics will be very much less in thecase of a tube containing mercury vapour than in the case of a tubecontaining say helium. Generally, it is referable that the length of thegap be r uced to the smallest racticable value, which length will belimited by constructional difiiculties. It is possible to obtain asmaller gap using a tube heated by, say, a thermionic discharge thanwhen a filament is used. Better results are obtained when both theelectrodes are allowed to become incandescent. When the device is usedwith both electrodes incandescent, rectification must be eected bymaking the anode at a suihciently low temperature so that it will notallow an inverse current to flow.

The use of this invention permits of the construction of vacuum tubeshaving hard characteristics in which the residual gas pressure iscomparatively high. The advantages resulting from this will be obtainedin several ways. Firstly, with ordinary vacuum tubes, it is necessary toevacute those intended for use as rectifiers or oscillators to a highvacuum of, say, a few hundredths of a'micron, and further it isnecessary by a lengthy and laborious process to heat treat theelectrodes, the valve envelope and the component parts to get rid of allabsorbed gases, so that these may not be evolved during operation of thetube. Not only is this an expensive process, but as the size of the tubeis increased it becomes increasingly difficult` to eect properly so thata limit is set to the size of the valve tube which may be constructed.When vacuum tubes are constructed according to this invention,'the heattreatment need not be so thorough since the pressure of the gas in thetube may be allowed to rise to comparatively high values before the tubebecomes soft` and as a result the costof production of such valves islessened; it also becomes possible to construct valves of larger sizes.Secondly, owing to the amount of gas evolved during operation, it is notpracticable to employ lordinary vacuum tubes having metallic containersor water cooled anodes. Bv the use of this invention, such devicesbecome practicable. Thirdly, it is not practicable to evacute anordinary valve tube during operation by means of a vacuum pump unless aliquid air or like trap is interposed between the vacuum tube and thepump owing to the diffusion of vapour from the pump into the tube. Forinstance. with a mercury pump, mercury vapour will diffuse into thevalve tube and will have a'pressure of about 1 micron atv atmospherictemperature. Mercury vapour' at this pressure would produce blue glow atanode voltages of 20 to 30 volts in va ves -constructed according topresent practice. By the use of this invention, vacuum tubes may beexhausted during operation by a vacuum pump such, for instance, as amercuryvapour pump without a low temperature trap being interposedbetween the pump and the vacuum tube; or alter.- natively where extrahigh voltages are applied to the tube a moderately coldtrap for exam le,brine' and ice may be employ 1. In a vacuum electricdischargetube'having an envelope containing an ionizable rarifiedatmosphere,- an anode, a thermionic cathode mounted within the anode,said an ode and cathode being so closely spaced that the inter-electrodeelectron paths are of less length than the mean free ath in said at-lmosphere and are suilicient y short to give the tube hardcharacteristics which atmosphere in a longer gap would -be ionized andwould give the tube soft characteristics, and means at the boundaries ofsaid cathode exposed to the space outside of the inter-electrode spaceto substantially confine the electrons to said inter-electrode space.

2. In a` vacuum lelectric discharge tube having an envelope containing arariied ionizable atmosphere, an anode, a cathode mounted within theanode and capable of emitting electrons only when heated above normaltemperature, said cathode and anode being s aced suiliciently closely toprovide paths or the electrons which are of less length than the meanfree path in said atmosphere and are suciently short to give the tubehard characteristics, the pressure of said atmosphere being so high thatthe electrons ina longer gap would produce gaseous ionization 1n suchatmosphere and would' give the tube soft characteristics, .and shieldsat the ends of the cathode for substantially confining the electrons tothev inter-electrode gap.

3. In a vacuumv electric discharge tube having an envelope containing anionizable raried atmos here, an anode, a thermionic electronemittingcathode mounted within the anode, said electrodes having their activesurfaces so disposed and closely spaced as to provide inter-electrodeelectronpaths which are of less length than the mean free path in saidatmosphere andare suiliciently short'to give the tube hardcharacteristics, which atmosphere in a longer gap would bc ionized bsuch electrons and would give the tube so t characteristics, andelectrostatic shields adjacent to the ends of the cathode tosubstantially confine the electrons to said inter-electrode space.

A.4. In a vacuum velectric discharge tube having an envelope containingan ionizable raried atmosphere, a substantially cylindrical anode, asubstantially cylindrical thermionic, electron-emitting cathode mountedwithin the. anode and a ranged within the ends of the anode, saidelectrodes having their active surfaces suiciently closely spaced toprovide inter-electrode electron paths all of which are shorter than themean free path insuch atmosphere and are suiciently short to give the'tube hard characteristics, said atmosphere being at such pressure aswould cause impact ionization in a longer gap and would give the tubesoft characteristics, and shields mounted within the anode and at the.respective ends of the cathode for preventlng the electrons escapingfrom said inter-electrode space and of following vpaths voutslde saldspace of suicient length to cause 1on1zat1on of said atmosphere outsidesaid space and to Ve the tube soft characteristics.

testimony whereof I have signed m name to this specification.

ERNEST YEOMAN ROBINSON.

